Release 6 of the WCDMA (Wideband Code Division Multiple Access) specification—for instance in prior art document 3GPP TS 25.309, “FDD Enhanced Uplink; Overall Description; Stage 2”, Version V6.6.0 of 2006-04-06—discloses a High Speed Uplink Packet Access (HSUPA), also called Enhanced Uplink, communication scheme. The HSUPA aims to match the bit rates provided by the downlink High Speed Data Packet Access (HSDPA) scheme, so as to cater for improved interactive, background and streaming services.
Relevant sections can moreover be found in 3GPP References TS25.211 and TS25.321—MAC protocol specification. 25.214 Physical Layer Procedures (FDD). 25.321 MAC protocol specification.
FIG. 1, a HSUPA network overview is indicated (HSDPA related channels are not included in the figure). The network comprises a Core Network communicating with a Radio Network Controller (RNC, S-RNC, D-RNC (Drifting-RNC)) over the lu interface, or Iur interface; a first base station, Node B, B1, a second base station, Node B, B2, both base stations comprising a EUL scheduler unit. The EUL Scheduler (EUL_SCH) is also denoted the MAC-e Scheduler, and communicating with the RNC over respective lub interfaces.
The following HSUPA channels are transmitted over the air interface; the E-AGCH to convey absolute grant signalling from the MAC-e scheduler towards the UEs, the E-RGCH for relative grant signalling, E-HICH to convey acknowledgement feedback from Node-B decoding of UE transmitted data, Dedicated Physical Channel (DPCH) or Fractional DPCH to convey Transmit Power Control (TPC) commands, Enhanced DPDCH (E-DPDCH) to convey the MAC-e payload and Enhanced DPCCH (E-DPCCH) to convey the control signalling of the MAC-e.
Node B1 corresponds to the serving cell in this example (E-AGCH is only transmitted from the serving cell) and node B2 corresponds to a non-serving cell.
Document 3GPP TS 25.309 FDD, Enhanced Uplink Overall description, mentioned above gives an overview of the Enhanced Uplink functionality.
An overview of the HSUPA can also be found in prior art document “High Speed Uplink Packet Access (HSUPA); White Paper, application note 1MA94”, Rohde Schwarz, January 2006, retrieved on the internet on 2011 Oct. 24.
According to the HSUPA specification, the Enhanced Dedicated Channel (E-DCH) high speed uplink transport channel offers a number of features such as: short Transmission Time Interval (TTI), Fast Hybrid Automatic Repeat Request (ARQ) with soft recombining, fast scheduling for reduced delays, increased data rates and increased capacity.
When a UE is setting up communication with a Node B, the setup procedure may be followed by a HSDPA session, for e.g. downloading/surfing an internet page using TCP. Depending on the capabilities of the user entity, this may moreover involve HSUPA transmissions whereby the Node B that transmits TCP messages on the HSDPA downlink channel will receive TCP acknowledgements on the E-DCH uplink to Node B. Since Node-B determines, or schedules, at which pace a UE shall transmit on E-DCH, Node-B utilises the E-AGCH to convey scheduling decisions. A shorter delay, measured from the time until a TCP data segment is sent downlink until a TCP acknowledgement is sent on the uplink as a response, leads to a decreased downloading time of file transfers etc. due to the shorter round trip time estimate of the TCP layer.
In order to use a HSUPA service with Node B, the user entity is informed about which E-AGCH code it is supposed to receive downlink traffic on. For this purpose, the E-AGCH, which is a shared channel within the cell, is used. The E-AGCH can be defined to have a number of one to several channelization codes.
E-AGCH channels are configured to a Node B in a configuration or re-configuration procedure with the RNC via the NBAP (Node B Application Part) signalling protocol.
HSUPA is similar in many respects to HSDPA. However, unlike HSDPA, HSUPA does not utilize a shared channel for data transfer in the uplink. In W-CDMA, each UE already uses a unique scrambling code in the uplink so each UE already has a dedicated uplink connection to the network with more than ample code channel space in that connection. This is in contrast to the downlink where the Node B uses a single scrambling code and then assigns different OVSF channelization codes to different UE's. The shared resource in the uplink is actually the interference level at the Node B, which the network manages through the fast closed loop power control algorithm. The fact that the UE has a dedicated connection to the network in the uplink influences the design of HSUPA quite considerably. The goals of HSUPA were to support fast scheduling (which allows the network to rapidly effect a change of which UE's that should transmit and at what rate) and to reduce the overall transmission delay. Transmission delay reduction is achieved through fast HARQ (hybrid automatic repeat request) retransmissions, in a manner very similar to HSDPA and at an optional shorter 2 ms TTI. As the primary shared resource on the uplink is the total power arriving at the base station, HSUPA scheduling is performed by directly controlling the maximum amount of power that a UE can use to transmit with at any given point in time.
The network has two methods for controlling the UE's transmit power on the E-DPDCH; it can either use a non-scheduled grant or a scheduled grant. In the non-scheduled grant the network simply tells the UE the maximum block size that it can transmit on the E-DCH during a TTI. This block size is signalled at call setup and the UE can then transmit a block of that size or less in each TTI until the call ends or the network modifies the non-scheduled grant via an RRC reconfiguration procedure. The block size deterministically maps to a power level, which is also configured by the network during call setup. The non-scheduling grant is most suited for constant-rate delay-sensitive application such as voice-over-IP.
Regarding the scheduled grant, the UE maintains a Serving Grant that it updates based on information received from the network. The Serving Grant directly specifies the maximum power that the UE can use on the E-DPDCH in the current TTI. As E-DCH block sizes map deterministically to power levels, the UE can translate its Serving Grant to the maximum E-DCH block size it can use in a TTI (the mapping of power levels is determined by the E-TFCI (The E-TFCI (E-DCH Transport Format Combination Identifier) includes information about the transport block set size, which is related to the data rate) Reference Power Offsets that are signalled at call setup).
There are two ways the network can control the UE's Serving Grant. The first is through an absolute grant, transmitted on the shared E-AGCH downlink channel, which signals a specific, absolute number for the Serving Grant. The other way is through relative grants, transmitted using the downlink E-RGCH channels, that incrementally adjust a UE's Serving Grant up or down from its current value. At any given point in time, the UE will be listening to a single E-AGCH from its serving cell and to one or more E-RGCH's. The E-AGCH is a shared channel so the UE will only update its Serving Grant if it receives a block on the E-AGCH that is destined for it (the E-RNTI identity signalled at call setup is used on the E-AGCH to direct transmissions to particular UE's). The E-AGCH transmission contains an Absolute Grant Value and an Absolute Grant Scope. The value corresponds to a maximum rate and the scope can be set to either “all HARQ processes” or “per HARQ process”, c.f. 3GPP 25.321. The E-RGCH is also shared by multiple UEs, but on this channel the UE is listening for a particular orthogonal signature rather than a higher layer identity. If it does not detect its signature in a given TTI, it interprets this as a “Hold” command and thus makes no change to its Serving Grant.
The Node B MAC-e Scheduler issues absolute grants on the downlink E-AGCH channel, that is, messages which grant the user entity the right to transmit at given bit rates on the uplink. Since bandwidth needs vary dynamically over time, it is desirable that the power emissions by user entities are regulated speedily so that bandwidth is not unnecessarily wasted. User entities transmit requests as Happy/Not Happy concerning their need for higher speeds.
There is a risk that an UE falsely detects an absolute grant that was not transmitted, also called a ghost grant. Such a ghost grant may cause the UE to transmit on a rate that NodeB is not ready to receive at.
The inventors have found that according to the current standardized specification, Node B will in such situation, where a ghost grant is detected by the UE; send a NACK on E-HICH. It will also send a new Absolute Grant to avoid the problem for new UE transmissions. However, the 3GPP standard allows for the UE to re-transmit at a previous grant level, but since Node B still has no resources to decode transmissions, it will also fail and a new NACK is sent. This will go on until the maximum allowed number of re-transmissions are reached (e.g. 7) which will cause a substantial amount of interference in the cell. Eventually, the UE will give up and stop transmitting, which subsequently will lead to an upper layer RLC (Radio Link Control layer) re-transmission.
According to the inventors, the risk for false detection of E-AGCH, for a 2 ms TTI UE can be assessed as follows: In one minute there are 30.000 TTIs. With 16 bit CRC (Cyclic Redundancy Check) there are 65.536 combinations. In lab and field tests one typically experiences a false E-AGCH (E-AGCH: E-DCH Absolute Grant Channel) detection in a UE in the interval of a few minutes.
In FIG. 2, an exemplary situation—as perceived by the inventors—is shown where a UE still has data to transmit but where the UE has not been awarded any grant. By way of example, the UE detects in 11 a ghost grant at HARQ process 4 that is valid for all HARQs, which seemingly makes it possible to transmit on E-TFCI 84. NodeB detects the problem and issues a new grant in 12 for deactivating all HARQ's. In the example, Node B does however not has the ability to decode the unexpected UE transmission so Node B additionally responds by transmitting a NACK. This pattern continues for all 8 HARQ's with initial transmission, RSN (Retransmit Sequence Number)=0. Thereafter, further re-transmissions follow and the pattern can continue for up to 7 re-transmissions for 2 ms TTI configuration. It is noted that the Node B manufacturer/operator may choose the maximum number of possible retransmissions and that longer maximum retransmission numbers aggravate the problem. At this point in time, the procedure stops, 13, since UE has reached a maximum number of retransmissions and must abort those transmissions. New transmissions must follow that last received grant, which is the one received at 12 that do not allow any more transmissions.
The inventors have found that ghost grants cause serious problems for TD (Time Division) scheduling. Under TD scheduling, the UE has only a grant on one or a few HARQ processes, while the other HARQ processes are deactivated from scheduled data. If a ghost grant valid for all HARQs is detected in the UE, the UE suddenly starts to transmit on all HARQ process and sometimes at a very high rate. Even though there may be no resources available in Node B to decode the transmissions, the UE continues up to 8 HARQ cycles, i.e. 8*16 ms.
The problem is predominant for TD scheduling, since if a UE has a grant in only 1 HARQ out of 8, then processing resources can not be reserved for all 8.